Metabolism of 1- and 3-Fluorobenzo[a]pyrene by Cytochrome p450 and Horseradish Peroxidase

Abstract

Benzo[a]pyrene (BP) is a good model for elucidating the mechanism of oxygen transfer for substrates that are considered good electron donors. Fluoro substitution of BP represents a suitable probe for studying mechanisms of oxygen transfer in the metabolic formation of BP quinones and BP phenols. By using this strategy with 6-fluoroBP (6-FBP), we have previously demonstrated that the BP quinones are formed metabolically via an initial one-electron oxidation of BP catalyzed by cytochrome P450. Now we have synthesized 1-FBP and 3-FBP with the purpose of elucidating the mechanism of phenol formation. If formation of 3-hydroxyBP (3-OHBP) (major metabolite of BP) and 1-OHBP (minor metabolite of BP) occurs via an initial electron transfer from BP to cytochrome P450, when we use 3-FBP and 1-FBP as substrates, one of the metabolites formed should be BP-3,6-dione and BP-1,6-dione, respectively, with displacement of fluorine. Metabolism of 1-FBP and 3-FBP by rat liver microsomes and by horseradish peroxidase (HRP) was conducted. With 1-FBP, BP-1,6-dione was obtained with cytochrome P450 or HRP. In addition, with cytochrome P450 the 4,5-, 7,8- and 9,10-dihydrodiol and a phenol of 1-FBP were obtained. With 3-FBP as substrate, HRP produced only BP-3,6-dione, whereas cytochrome P450 produced BP-3,6-dione, the 4,5-, 7,8- and 9,10-dihydrodiol, and a phenol of 3-FBP. The formation of BP quinones from 1-FBP and 3-FBP clearly demonstrates that 3-OHBP and 1-OHBP in the metabolism of BP are produced via an initial electron transfer from BP to the activated intermediate of cytochrome P450 or HRP.

Key Words:

• Metabolism
• cytochrome P450
• horseradish peroxidase
• catalytic mechanism
• oxygen transfer
• electron transfer